Stent having circumferentially deformable struts

ABSTRACT

Disclosed is a method of treating a bodily lumen with a stent, the method comprising: disposing a stent within a bodily lumen, the stent comprising a plurality of deformable struts that are substantially circumferentially aligned and are configured to selectively deform in a circumferential direction in localized regions in the struts upon application of an outward radial force; and expanding the stent by applying the outward radial force, wherein the outward radial force causes selective deformation of the deformable struts in a localized region in the struts.

This application is a continuation of application Ser. No. 11/823,954filed on Jun. 29, 2007 and is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Disclosed herein is a stent that is radially deformed in vivo afterimplantation of the stent in a bodily lumen.

2. Description of the State of the Art

This invention relates to radially expandable endoprostheses, which areadapted to be implanted in a bodily lumen. An “endoprosthesis”corresponds to an artificial stent that is placed inside the body. A“lumen” refers to a cavity of a tubular organ such as a blood vessel.

A stent is an example of such an endoprosthesis. Stents are generallycylindrically-shaped devices, which function to hold open and sometimesexpand a segment of a blood vessel or other anatomical lumen. Stents areoften used in the treatment of atherosclerotic stenosis in bloodvessels. “Stenosis” refers to a narrowing or constriction of thecross-sectional area of a bodily passage or orifice. In such treatments,stents reinforce body vessels and prevent restenosis followingangioplasty in the vascular system. “Restenosis” refers to thereoccurrence of stenosis in a blood vessel or heart valve after it hasbeen treated (as by balloon angioplasty, stenting, or valvuloplasty)with apparent success.

The treatment of a diseased site or lesion with a stent involves bothdelivery and deployment of the stent. “Delivery” refers to introducingand transporting the stent through a bodily lumen to a region, such as alesion, in a vessel that requires treatment. “Deployment” corresponds tothe expanding of the stent within the lumen at the treatment region.Delivery and deployment of a stent are accomplished by positioning thestent about one end of a catheter, inserting the end of the catheterthrough the skin into a bodily lumen, advancing the catheter in thebodily lumen to a desired treatment location, expanding the stent at thetreatment location, and removing the catheter from the lumen. In thecase of a balloon expandable stent, the stent is mounted about a balloondisposed on the catheter. Mounting the stent typically involvescompressing or crimping the stent onto the balloon. The stent is thenexpanded by inflating the balloon. The balloon may then be deflated andthe catheter withdrawn. In the case of a self-expanding stent, the stentmay be secured to the catheter via a retractable sheath or a sock. Whenthe stent is in a desired bodily location, the sheath may be withdrawnwhich allows the stent to self-expand.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Radial strength and rigidity,therefore, may also be described as, hoop or circumferential strengthand rigidity.

Additionally, the stent should also be longitudinally flexible to allowit to be maneuvered through a tortuous vascular path and to enable it toconform to a deployment site that may not be linear or may be subject toflexure. The material from which the stent is constructed must allow thestent to undergo expansion. Once expanded, the stent must maintain itssize and shape throughout its service life despite the various forcesthat may come to bear on it, including the cyclic loading induced by thebeating heart. Finally, the stent must be biocompatible so as not totrigger any adverse vascular responses.

The structure of a stent is typically composed of scaffolding thatincludes a pattern or network of interconnecting structural elements orstruts. The scaffolding can be formed from wires, tubes, or sheets ofmaterial rolled into a cylindrical shape. The scaffolding isconventionally designed so that the stent can be radially contracted (toallow crimping) and radially expanded (to allow deployment). Aconventional stent is allowed to expand and contract through movement ofindividual structural elements of a pattern with respect to each other.Such movement typically results in substantial deformation of localizedportions of the stent's structure. The pattern of the stent is designedto maintain the longitudinal flexibility and radial rigidity required ofthe stent. Longitudinal flexibility facilitates delivery of the stentand radial rigidity is needed to hold open a bodily lumen.

A stent may be constructed of polymeric material. Polymeric stentsrequire adequate circumferential strength and radial rigidity.Inadequate circumferential strength potentially contributes to arelatively high incidence of recoil of polymeric stents afterimplantation into vessels. The requirement of high strength and rigidityis seemingly at odds with the need for flexibility during delivery. Themovable structural elements in the stent pattern provide someflexibility.

One potential problem with polymeric stents is that their struts cancrack during crimping and expansion. The localized portions of the stentpattern subjected to substantial deformation tend to be the mostvulnerable to failure. Therefore, it would be desirable to have a methodof treating a bodily lumen with a polymeric stent in which the stent hasadequate flexibility during delivery, high radial strength and rigidityafter deployment, and that is relatively free of localized regions ofhigh deformation susceptible to failure.

SUMMARY OF THE INVENTION

Various embodiments of the present invention include a method oftreating a bodily lumen with a stent, the method comprising: disposing apolymer stent within a bodily lumen, wherein the stent comprises aplurality of circumferential rings composed of circumferentially alignedstruts connected at connection regions, wherein the circumferentialrings are connected by linking struts that connect adjacent rings atconnection regions on the adjacent rings, wherein each ring has at leastone deformable strut of the circumferentially aligned struts that has aregion of reduced cross-section formed by a notch in a portion of thedeformable strut between two connection regions, wherein the notchedportion formed in the at least one deformable strut is for inducingnecking in the deformable strut, wherein the deformable struts areconfigured to selectively deform in a circumferential direction inlocalized regions in the deformable struts upon application of anoutward radial force; and expanding the stent by applying the outwardradial force, wherein the outward radial force causes selectivedeformation comprising necking in a circumferential direction of thedeformable struts in the localized region in the struts, wherein theselective deformation is initiated at the reduced cross-section region,wherein the reduced cross-section region guides the necking in thecircumferential direction.

Further embodiments of the present invention include a polymer stentconfigured for being expanded by a balloon, comprising: a plurality ofrings composed of struts connected at connection regions, wherein all ofthe struts extend only in the circumferential direction, both before andafter being expanded by the balloon; linking struts connecting adjacentrings at the connection regions; at least one deformable strut for eachof the rings; and a notched portion formed in the at least onedeformable strut to induce necking in the deformable strut, whereuponexpansion of the stent the necking occurring at the notched portionenables the respective ring to increase in diameter and provide radialsupport for a bodily lumen.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1( a) depicts a stent having a plurality of deformable strutsaccording to one embodiment of the invention.

FIG. 1( b) depicts a close up view of a localized region having adeformable strut prior to deformation.

FIG. 1( c) depicts a stent having a plurality of deformed struts afterdeformation.

FIGS. 2( a)-(e) depict a deformable strut at various stages of selectivedeformation in a localized region.

FIG. 3 depicts a close up view of a portion of the stent according toFIG. 1( a) having circumferential rings that include deformable strutsand linking elements.

DETAILED DESCRIPTION OF THE INVENTION

The term “stent” as used herein is intended to include, but is notlimited to, balloon-expandable stents, stent-grafts, and vasculargrafts. In general, stents are composed of a pattern or network ofcircumferential and longitudinally extending interconnecting structuralelements or struts.

Processes to manufacture a stent may include extruding a polymeric tubefor use in fabricating a stent. The polymer can be subjected toprocessing that enhances the mechanical strength of the polymer. Thestent may then be crimped onto a delivery device to deliver the stentinto a lumen for treatment. Due to radial expansion and the shape-memoryof the polymers, polymer recoil may occur after the stent is deployedinto the lumen. Crimping and expansion of a stent having a typical stentpattern result in regions of the stent experiencing high deformationduring crimping and expansion. Thus, when a stent is crimped orexpanded, some portions of a stent pattern may have no or relatively nostrain, while others may have relatively high strain. Such strain on astent may lead to cracking during crimping, sterilization, and expansionor deployment of the stent into the body lumen.

The embodiments disclosed herein provide for certain advantagesregarding mechanical properties and stability. The deployment of thestent enhances the radial strength of the stent. Further, methodsdisclosed herein do not require the stent to be crimped to a smallerdiameter in order to be delivered into a bodily lumen. Since the stentformed according to the embodiments described herein need not becrimped, the stent is less likely to suffer cracking during deployment.

Embodiments provide for a polymeric stent having adequate flexibilityduring delivery as well as high radial strength and rigidity (highmodulus) after deployment. The stent can also be made to be relativelyfree of localized regions of high deformation caused by bending strutsthat are susceptible to failure, which occurs when struts bend

FIG. 1( a) depicts an axial projection of a stent 100 according to oneembodiment of the invention having a plurality of deformable struts 110,with A-A corresponding to a longitudinal axis of the stent. As depicted,deformable struts 110 may be substantially aligned in a circumferentialdirection B-B. Thus, upon application of outward radial force,deformable struts 110 are configured to selectively deform or neck indirection B-B. Stent 100 is radially expanded to deploy in vivo byapplying a radial force from a delivery implement, such as a ballooncatheter. Stent 100 expands through selective deformation or necking ofdeformable struts 110 in localized regions 130 of struts 110. FIG. 1( b)depicts a close up view of a localized region 130 of strut 110 having adeformable region 135 and recess 140.

As outward radial force is applied, stress is applied to deformablestruts 110 beyond the yield strength of the material, and necking occursin deformable regions 135 as depicted in FIG. 1( c). “Stress” refers toforce per unit area, as in the force acting through a small area withina plane. As a result of outward radial force on the stent 100, struts110 are deformed along B-B causing stent 100 to be radially expanded.

In one embodiment, the deformable region 135 having a recess serves asan initial deforming point in deformable struts 110 and guidesdeformation in the circumferential direction 120 upon application of theoutward radial force. As depicted, recess 140 can be formed by providinga region in deformable strut 110 having a smaller cross-sectional areathan the remaining portion of deformable strut 110. Recess 140 may beformed, for example, by laser cutting the recess into the stent. Therecess is not limited to the shape and size as depicted in FIGS. 1( a)and 1(b). Rather, recess 140 can be of any shape and size, so long asrecess 140 is configured result in selective deformation or necking uponapplication of stress in deformable region 135 of recess 140. Ingeneral, the recess of the stent may have a smaller circumferential orlower mass. It is also contemplated that other features can serve asinitial deforming points besides the use of a recess in the polymericmaterial, such as a notch or extension from the strut that initiatesselective deformation. Also, struts 110 can include a polymeric regionhaving a lower molecular weight as compared to other parts of the stent.Such regions can serve as initial deforming points in the deformablestruts upon the application of radial force.

When the stent is deployed, an application of an outward radial forcecauses the stent to deform and radially expand. FIG. 1( c) depicts astent having a plurality of deformed struts in the expanded condition.The outward radial force applied by a delivery implement causesdeformable strut 110 to neck or elongate in localized regions 130 of thestent due to the recesses 140. As a result of the deformation ofdeformable struts 110, the entire stent 100 is radially expanded.

As depicted in FIG. 2, selective deformation upon application of radialforce causes a deformed region 135 to deform resulting in a smallercross-sectional area. Regions 142 on either side of recess 140 mayexperience deformation and decrease in cross-sectional area. Thus, ineffect, deformable region 135 having recess 140 is elongated such thatnecking occurs in the polymeric material. Necking in deformable struts110 in regions of struts with recesses 140 cause radial expansion orlengthening of deformable struts 110. Thus, recess 140 in deformablestrut 110 serves as an initial deforming point that guides deformationas the stent is deformed circumferentially upon deployment. Thesubstantially circumferentially aligned strut 110 having at least onerecess 140 may be caused to deform upon expansion and deployment of astent. Deformable strut 110 may be configured to deform at recess 140upon application of radial force in the circumferential direction 250,causing necking of the polymeric material. Necking is induced in region135 as depicted in FIG. 2( b), which causes deformed portion of region135 to elongate to a longer deformed portion 220 upon furtherapplication of radial force. Eventually, a second deformed portion 230is formed as depicted in FIG. 2( d). The second deformed portion 230elongates upon further application of radial force as depicted in FIG.2( e).

In one embodiment, the stent is expanded to a circumference that is twoto six times its original circumference by expansion of the deformablestruts. More narrowly, the stent is expanded to a circumference that istwo to four times its original circumference by expansion of thedeformable struts. In one embodiment an outer diameter for a crimpedstent is about 1.0 mm, and an outer diameter for the stent in theexpanded state is about 3 mm.

Upon deformation of the deformable struts, the thickness of the deformedportions are reduced. After deployment, the tensile strength of thestent is also increased upon radial expansion of the stent duringdeployment.

The selectively deformed regions can exhibit an increase incrystallization and induced alignment of polymer chains along thecircumferential direction. The crystals generated by circumferentialdeformation serve as an intrinsic locking mechanism. The inducedcrystallization and alignment increases the strength and the modulusalong the deformation, or necked region. As a result, the radialstrength and shape stability of the stent are increased.

FIG. 3 depicts a close up view of a portion 300 of the stent accordingto FIG. 1( a) having circumferential rings 310 that includesubstantially circumferentially aligned, deformable struts 110 joined bylinking struts 320. Deformable struts 110 having a length L, straightportions L1 of deformable struts, and linking struts 320 are depicted.Both struts 310 and 320 have a thickness T. Deformable strut length L, alength L1, and a wall thickness T of the linking elements 320 can bevaried to obtain desired mechanical properties in the stent, such asradial strength and flexibility of the stent. Numerous embodiments arepossible by varying the parameters of the stent. In addition,embodiments are not limited to the patterns depicted in FIG. 1.

FIG. 3 also illustrates that width of strut W2 at recess 140 which issmaller than the width W1 of the unrecessed portion of strut 110. In oneembodiment, width W1 may be from about 0.010 to about 0.040 inches, morenarrowly from about 0.020 to about 0.030 inches, or even more narrowlyfrom about 0.022 to about 0.030 inches.

The width W2 at recess 140 may range from about 0.005 to about 0.009inches, or more narrowly from about 0.005 to about 0.008 inches.

In one embodiment, wall thickness T may range from about 0.005 to about0.009 inches, or more narrowly about 0.006 to about 0.008 inches.

Any number of parameters for the wall thickness T, strut width W2 atrecess and strut width W1 at remaining portions of strut can vary.

The number of circumferential rings 310 around the circumference of thestent may also vary. In one embodiment, as depicted in FIG. 1( a), stent100 has 6 circumferential rings.

By varying the length L of deformable strut, a desired radial expansionpercentage may be achieved. Deformable strut length L describes thedistance between the diamonds on the pattern. A connection section Cdescribes the distance of the diamond-shaped connection section of thestruts. In one embodiment, connection section C as depicted in FIG. 3varies from about 0.008 inches to about 0.020 inches, more narrowly fromabout 0.010 inches to about 0.018 inches, and even more narrowly fromabout 0.011 inches to about 0.015 inches. These diamonds will stopdeformation (expansion) of the strut. By adjusting the distance betweenthe diamonds, it is possible to vary the amount of radial expansionallowed in the stent. In one embodiment, selective deformation causes aradial expansion of the stent to vary from 300% to 600% after deploymentof the stent. It should be noted that the stent may encompass otherdesigns not limited to the design depicted in FIG. 1 or FIG. 3.

Selective deformation of the deformable struts may inducecrystallization and/or circumferential molecular orientation in thepolymer. The polymer may crystallize during selective deformation toform crystals or net points that serve as intrinsic locking mechanismsthat increase the mechanical property of the stent and stabilize thestent after radial expansion. In one embodiment, the selectivedeformation of the deformable struts increases the crystallinity in thepolymer of the deformable struts to a crystallinity between about 10% toabout 50%, more narrowly between about 20% to about 40%, and morenarrowly between about 30% to about 40%.

As mentioned above, stents are conventionally radially expanded in orderto impart mechanical properties in the stent to strengthen the stent.Such conventional methods also include crimping the radially expandedstent in order to deliver the stent into a bodily lumen beforedeployment into the lumen. In contrast, the embodiments disclosed hereinprovide for radial expansion in a bodily lumen by selective deformationin the lumen. Thus, mechanical properties are sufficiently imparted intothe stent during deployment of the stent. Selective deformation of thedeformable struts causes molecular orientation in the polymer andstabilizes the stent, thereby increasing mechanical strength of thedeployed stent. In conventional stents, radial expansion is due tobending of curved portions of stents. Such stents are typically crimpedfrom a fabricated diameter to a delivery diameter. Such bending portionsare susceptible to cracking due to the crimping and expansion.

The stent need not be crimped prior to implantation. Since radialexpansion is due to deformation of stents aligned in the circumferentialdirection, embodiments of the stent are fabricated at a deliverydiameter. Thus, stent need not be crimped in order to deliver the stentinto the lumen, but is rather implanted into the lumen in the unexpandedstate as depicted in FIG. 1( a). Because the stent need not be crimpedin order to deliver the stent into the lumen, the stent can be made tobe free or relatively free of localized regions of high deformationcaused by bending struts that are susceptible to failure.

Chemical and mechanical properties such as strength, modulus, and T_(g)may be modified by inducing molecular orientation and crystallinity inthe polymer. As mentioned above, the stent may be radially expanded anddeployed in the bodily lumen. The stent is radially expanded bycircumferentially deforming the deformable struts. Once expanded, thedeformed struts expand the lumen and may support the lumen. The stentmay be configured such that the deformable struts are expandedsubstantially uniformly around its circumference. Therefore, the radialstrength and rigidity of the deformed struts may be increased byexpansion or deployment of the stent in the lumen.

The degree of polymer chain alignment induced by stress upon deploymentalso depends on the temperature of the polymer when stress is applied toexpand the stent. Below the T_(g) of a polymer, polymer segments may nothave sufficient energy to move past one another. Above T_(g) of thepolymer, polymer chain alignment may be readily induced with appliedstress since rotation of polymer chains, and hence segmental mobilityoccurs relatively easily. Between T_(g) and the melting temperature ofthe polymer, T_(m), rotational barriers exist. However, the barriers arenot great enough to substantially prevent segmental mobility. As thetemperature of a polymer is increased above T_(g), the energy barriersto rotation decrease and segmental mobility of polymer chains tends toincrease. As a result, as the temperature increases, polymer chainalignment is more easily induced with applied stress.

In one embodiment, the polymer of the stent is near its T_(g) duringdelivery of the stent. In some embodiments, the polymer of the stent hasa T_(g) at or below body temperature (37° C.), such that the stent isheated to the polymer's T_(g) by inserting the stent into the body. Someembodiments may include heating the stent prior to delivering the stentinto a body. Other embodiments may include heating the stent duringdelivery and/or during expansion of the stent.

After expansion in a lumen, it is generally desirable for a stent toremain rigid and maintain its expanded shape so that it may continue tohold open the lumen. In one embodiment, induced orientation andcrystallization of the deformed regions of the struts after expansion ordeployment of the stent increases the T_(g) of at least a portion of thedeformed struts. Thus, the T_(g) of the polymer in the stent may beincreased after expansion to above body temperature. Barriers to polymerchain mobility below T_(g) inhibit or prevent loss of inducedorientation and crystallization.

In one embodiment, the deformable struts include a semicrystallinepolymeric material having a glass transition temperature that is below37° C. before necking and above 37° C. after necking. In such anembodiment, the crystallinity of the deformable struts can be at least30% after necking.

Additionally, it is desirable for a stent to have sufficient flexibilityduring delivery. Embodiments of the method described above allow a stentbe flexible during delivery as well as be strong and rigid afterdeployment. The expansion of the stent and consequential necking thattakes place during expansion of the stent which induces strength andrigidity is performed only after the stent is delivered to a desiredlocation where it is expanded.

In some embodiments, the deformed struts are expanded plastically upondeployment, allowing the stent to retain its expanded shape upondeployment. In other embodiments, the deformed struts are elasticallyexpanded. However, even an elastically expanded stent may retain itsexpanded shape if its temperature is below its T_(g). Thus, applyingstress-induced crystallization to the polymeric deformed strut mayresult in a permanently deformed high strength, high modulus materialwith a higher T_(g) than the original polymeric strut prior todeformation. The term plastic deformation refers to permanentdeformation that occurs in a material under stress after elastic limitshave been exceeded. “Elastic limit” refers to the maximum stress that amaterial will withstand without permanent deformation.

To fabricate the stent, the tube is laser cut into a stent pattern thatincludes at least one recess in the plurality of substantiallycircumferentially aligned deformable struts. In one embodiment, a recessis included on every other circumferentially aligned strut. In anotherembodiment, a recess is included on every circumferentially alignedstrut. In some embodiments, more then one recess is included on acircumferentially aligned strut. As mentioned above, the stent patternmay include any number of circumferentially aligned struts.

In one embodiment, the inner diameter of stent is slightly larger than adelivery implement's outer diameter to facilitate mounting the stent onthe delivery device. In another embodiment, the inside cross-sectionalarea of the stent prior to expansion may be equal to an outside diameterof the delivery implement. The diameter of a stent is increasedsubstantially or completely through circumferential deformation of thedeformable struts rather than through changes in angles betweenstructural elements.

A tube for use in fabricating the stent with desirable stent wallthickness may be extruded by a melt or solution process.

The stent may be coated with a drug, a polymer, or a polymer thatincludes a drug. The stent may be coated using any conventional stentcoating method. The stent may be mounted on a delivery implement such asa balloon-catheter assembly. The delivery implement is capable ofradially expanding the deformable struts in vivo.

The stent assembly may be sterilized by e-beam or other sterilizationmethods as known by those skilled in the art.

A stent made from a biodegradable polymer is intended to remain in thebody for a duration of time until its intended function of, for example,maintaining vascular patency and/or drug delivery is accomplished. Afterthe process of degradation, erosion, absorption, and/or resorption hasbeen completed, no portion of the biodegradable stent, or abiodegradable portion of the stent will remain. In some embodiments,very negligible traces or residue may be left behind.

The polymer used to fabricate the stent can be biostable, bioabsorbable,biodegradable, or bioerodable. Biostable refers to polymers that are notbiodegradable. The terms biodegradable, bioabsorbable, and bioerodable,as well as degraded, eroded, and absorbed, are used interchangeably andrefer to polymers that are capable of being completely eroded orabsorbed when exposed to bodily fluids such as blood and can begradually resorbed, absorbed and/or eliminated by the body.

The stent may be fabricated from a blend of polymers. Representativeexamples of polymers that may be used to fabricate the stent or portionsof the stent, such as the deformable struts include, but are not limitedto, polymers selected from the group consisting of poly(glycolic acid),90:10 poly(glycolic acid) and lactic acid, or mixtures thereof. Otherpolymers are also contemplated for use in fabricating the stent orportions of the stent, such as the deformable struts, will be discussedin detail below. In one embodiment, deformable struts include a polymerthat is semicrystalline having a Tg in the range of 32° C. to 44° C. inthe amorphous phase. The orientation that is imparted into the strutduring deformation can increase the Tg of the deformed struts by up to10° C. The increase in Tg of the deformed struts provides forimplantation of a device having a Tg below body temperature atimplantation of the device, and a Tg that is increased above bodytemperature during deployment of the device. In one embodiment, themodulus of a polymer can increase by 5 orders of magnitude as thepolymer goes from below Tg to above Tg. As a result, a flexible,non-rigid material of the device turns into a more rigid material due tothe expansion experienced during deployment of the device.

The T_(g) of the blend may be tuned by adjusting the relative weightpercent of the components in the blend. Representative examples ofpolymers for use in fabricating the deformable struts or other parts ofthe stent may include, but is not limited to, poly(N-acetylglucosamine)(Chitin), Chitosan, poly(3-hydroxyvalerate), poly(lactide-co-glycolide),poly(3-hydroxybutyrate), poly(4-hydroxybutyrate),poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyorthoester,polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lacticacid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide),poly(L-lactide-co-D,L-lactide), poly(caprolactone),poly(L-lactide-co-caprolactone), poly(D,L-lactide-co-caprolactone),poly(glycolide-co-caprolactone), poly(trimethylene carbonate), polyesteramide, poly(glycolic acid-co-trimethylene carbonate),co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules(such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronicacid), polyurethanes, silicones, polyesters, polyolefins,polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymersand copolymers other than polyacrylates, vinyl halide polymers andcopolymers (such as polyvinyl chloride), polyvinyl ethers (such aspolyvinyl methyl ether), polyvinylidene halides (such as polyvinylidenechloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics(such as polystyrene), polyvinyl esters (such as polyvinyl acetate),acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, andcarboxymethyl cellulose. Additional representative examples of polymersthat may be especially well suited for use in fabricating embodiments ofstents disclosed herein include ethylene vinyl alcohol copolymer(commonly known by the generic name EVOH or by the trade name EVAL),poly(butyl methacrylate), poly(vinylidene fluoride-co-hexafluoropropene)(e.g., SOLEF 21508, available from Solvay Solexis PVDF, Thorofare,N.J.), polyvinylidene fluoride (otherwise known as KYNAR, available fromATOFINA Chemicals, Philadelphia, Pa.), ethylene-vinyl acetatecopolymers, poly(vinyl acetate), styrene-isobutylene-styrene triblockcopolymers, and polyethylene glycol.

EXAMPLES

For EXAMPLE 1, 2, and 3, a stent pattern having 6 circumferential ringsas depicted in FIG. 1( a) was formed of a polymeric mixture of 90%glycolide and 10% poly L-lactide.

The width of the strut W1 was calculated based on mass balance.

Before expanded, the width of the strut W1=(length of straight portionL1*thickness T*strut width W1)=(0.0142 in*0.006 in*strut width W1).

After expanded, the width of the strut W1=(length of straight portionL1*thickness T*strut width at recess W2).

Therefore, the width of the strut W1=(0.006 in*0.006 in*0.050 in)/(0.006in*0.0142 in)=0.0214 in. Using this value of the strut width, the strutdesign will be calculated to be (L1*T*W1), or (0.0142 in*0.006 in*0.0214in).

Example 2

If the outer diameter of the stent was 0.048 inches, then the length ofstraight portion of deformable strut L1=[(Π*D)/6−0.012]=0.012 inches. Toreach the end product target (deployed stent), W1−0.025 in. The stentstrut design 0.012*0.006 in*0.025 in.

Example 3

A stent having a strut thickness W2 at recesses of 0.005 inches, adeployed inner diameter D of 3.5 mm, and a deployed outer diameter of3.75 mm was formed.

L=(Π*D)/6=0.23 in*Π=0.072 in.

L1=0.072−0.0012=0.060 in.

When the stent has an outer diameter of 0.046 in, L=(Π*D)/6=0.024 in.L1=(Π*D)/6 in−0.012=0.012 in.

The width of the strut W1 was calculated based on mass balance.

Before expanded, the width of the strut W1=(thickness T*length*strutwidth)=0.005*0.012*W1.

After expanded, the width of the strut W1=(thickness T*widthW2*L1)=0.005*0.006*0.060. Thus, W1=0.03 in.

Therefore, the stent design was fabricated to be equal to the product of(0.005 in)(0.012 in)(0.030 in).

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

The invention claimed is:
 1. A polymer stent configured for beingexpanded by a balloon, comprising: a plurality of rings composed of ringstruts connected at connection regions, wherein all of the ring strutsextend only in the circumferential direction, both before and afterbeing expanded by the balloon; and linking struts connecting adjacentrings at the connection regions; wherein, for each of the rings, thereis at least one deformable strut among the ring struts; and a notchedportion is formed in the at least one deformable strut to induce neckingin the deformable strut, the notched portion is spaced apart from theconnection regions, whereupon radial expansion of the stent the neckingoccurring at the notched portion enables the respective ring to increasein diameter and provide radial support for a bodily lumen.
 2. Thepolymer stent of claim 1, wherein the notched portion is a pair ofrounded notches formed on opposing sides of the at least one deformablestrut and a narrow strut portion disposed between the rounded notches,wherein the narrow strut portion is configured to neck when the stent isexpanded by the balloon.
 3. The polymer stent of claim 1, wherein thedistance between ring struts of adjacent rings is greater than thewidths of the ring struts.
 4. The polymer stent of claim 1, wherein eachof the connection regions has a longitudinal width greater than alongitudinal width of the at least one deformable strut.
 5. The polymerstent of claim 1, wherein each notched portion is spaced apart from theconnection regions by distances greater than a longitudinal width of thedeformable strut.
 6. The polymer stent of claim 1, wherein, for each ofthe rings, the at least one deformable strut includes a deformable strutconnected by one of the connection regions to one of the ring struts,and the ring strut connected to the deformable strut has no notchedportion.
 7. The polymer stent of claim 1, wherein when the stent isradially expanded, the notched portion deforms to a longer length in thecircumferential direction and a smaller cross-sectional area.
 8. Thepolymer stent of claim 7, wherein the deformation of the notched portionto a longer length in the circumferential direction allows the stent toexpand in circumference by an expansion factor in the range of two tosix.
 9. The polymer stent of claim 1, wherein the expansion factor is inthe range of two to four.
 10. The polymer stent of claim 1, wherein whenthe stent is radially expanded, crystallization and/or circumferentialmolecular orientation is/are induced in the notched portion.
 11. Thepolymer stent of claim 2, wherein each of the connection regions has alongitudinal width greater than a longitudinal width of the narrow strutportion.
 12. The polymer stent of claim 2, wherein each notched portionis spaced apart from the connection regions by distances greater than alongitudinal width of the deformable strut.
 13. The polymer stent ofclaim 2, wherein, for each of the rings, the at least one deformablestrut includes a deformable strut connected by one of the connectionregions to one of the ring struts, and the ring strut connected to thedeformable strut has no notched portion.
 14. The polymer stent of claim2, wherein when the stent is radially expanded, the narrow strut portiondeforms to a longer length in the circumferential direction and asmaller cross-sectional area.
 15. The polymer stent of claim 14, whereinthe deformation of the narrow strut portion to a longer length in thecircumferential direction allows the stent to expand in circumference byan expansion factor in the range of two to six.
 16. The polymer stent ofclaim 15, wherein the expansion factor is in the range of two to four.17. The polymer stent of claim 2, wherein when the stent is radiallyexpanded, crystallization and/or circumferential molecular orientationis/are induced in the narrow strut portion.
 18. The polymer stent ofclaim 2, wherein rounded notches are formed into sidewalls of the atleast one deformable strut.